phantomjinx/modeshape
ModeShape is a distributed, hierarchical, transactional, and consistent data store with support for queries, full-text search, events, versioning, references, and flexible and dynamic schemas. It is very fast, highly available, extremely scalable, and it is 100% open source and written in Java. Clients use the JSR-283 standard Java API for content repositories (aka, JCR) or ModeShape's REST API, and can query content through JDBC and SQL.